This invention relates to a pressure fluid operated percussive tool comprising a housing, a cylinder reciprocable in the housing, a piston hammer reciprocable in the cylinder and arranged to impact upon an anvil element coupled to a work implement, e.g. a chisel or a rock drill, a constantly pressurized chamber between the housing and the cylinder for biasing the cylinder forwardly in the housing, a first chamber for urging the piston hammer rearwardly in the cylinder, a periodically pressurized and vented second chamber between the piston hammer and the cylinder for urging the piston forwardly in the cylinder, and valve means for controlling the supply and venting of said second chamber in response to the axial position of the cylinder in the housing.
A percussive tool of this kind is described in U.S. Pat. No. 3,060,894, and it has an anvil element that extends into a periodically pressurized chamber. The rear end of a drill string constitutes the anvil element. Thus, the periodic fluid pressure results in a periodic force on the drill string and the forward force on the drill string will be unnecessarily high. This is a great disadvantage since it increases the feed force that must be applied to the housing of the tool. This feed force applied to the housing must balance the sum of the force exerted by the work implement on the work, for example on the pavement to be broken, and the internal reaction forces to the driving of the piston hammer. The feed force is usually the dimensioning factor and this is particularly true for hand held percussive tools.
In the percussive tool described in U.S. Pat. No. 3,060,894, there is no feed back from the piston to the valve, that is, to the cylinder that also forms a valve. Thus, there is a risk that the piston hammer and the cylinder run out of pace with each other particularly if the piston hammer receives a great rebounce energy when hitting the anvil, which occasionally occurs. It is an object of the invention to provide a pressure fluid operated vibration damped percussive tool in which the ratio between the impact power and the necessary feed force is high. It is also an object to provide a percussive tool of this kind in which the piston hammer and the cylinder do not tend to run out of pace with each other.